1. Field of the Invention
This invention relates to an optical system that is illuminated with light of defined polarization (for example, linearly polarized), in which a locally varying disturbance of the polarization occurs, due among other things to the degree of reflection at the inclined optical boundary surfaces, dependent on the direction of polarization, and to stress birefringence in lenses. After a following analyzer, local variations in brightness appear.
2. Discussion of Relevant Art
A system in which this effect is of importance is projection illumination equipment for microlithography, with a linearly polarized excimer laser in the DUV and a catadioptric reducing objective with a polarization beam splitter, as is known, for example, from German Patent DE 196 16 922 (U.S. Ser. No. 08/845,384 filed Apr. 15, 1997), the said patent being fully incorporated herein by reference.
Compensators are known for the continuously variable rotation of the direction of polarization, e.g. in the form of the Soleil-Babinet compensator, but of course only with uniform action over the whole light beam cross section.
Phase correction plates for wavefront correction in precision optics are known for use in microlithographic projection illumination equipment, and are for example machined with a "nanosphere" fine profile by ion beam etching.
Birefringent crystals of magnesium fluoride and of quartz are suitable for the production of polarization optical elements with transmission in the deep ultraviolet (DUV) at, e.g., 193 nm. Besides this, elements with stress birefringence are also known, e.g. according to German Patent DE 196 37 563.